The invention generally relates to the monitoring of electrode spacing in semiconductor wafer reaction chambers and, more particularly, to a non-intrusive technique which does not require that the chamber be opened for direct measurement using gauges or for indirect measurement using depositions upon monitor wafers.
Interelectrode gap spacing is a critical parameter in certain reaction chambers such as, for example, plasma enhanced chemical vapor deposition (PECVD) chambers. The parallelism and spacing of the plate electrodes in PECVD chambers affects the physics and the chemistry of the plasma discharge due to changes in the power density, residence time, etc., and strongly impacts the uniformity of the film thickness deposited across the wafers placed therein. When the chamber is in continuous use, the preestablished electrode spacing and parallelism may drift with time away from the desired settings. Consequently, the electrode spacing and parallelism needs to be checked periodically. It also has been noted that the interelectrode gap is sometimes inadvertently disturbed when routine or other maintenance is done on the chamber.
Conventional procedures used for checking the electrode spacing and parallelism require that the chamber be opened to allow the insertion of a calibrated spacing bar. This approach involves a cooling cycle and purging the system before the chamber can be opened. An alternative technique utilizes the running of a spacing matrix where the spacing is varied in small increments, e.g., 5 mil increments over a range of 20 mils, and the resulting film uniformity is observed on monitor wafers. The latter method requires the use of a fair number of monitor wafers which is an added expense. Both methods typically can take up to four hours, not including the time necessary to pump all atmospheric contaminants out of the system again, and bring the chamber to thermal equilibrium at the desired process temperature.